Our invention relates generally to cathode-ray tubes (CRTs) and particularly to a deflection system in a CRT. More particularly, our invention pertains to a traveling-wave deflection system of the type having a pair of helical conductors wound around core structures for deflecting a beam of electrons in a desired direction.
A typical conventional means for deflecting a beam of electrons in a given direction in a CRT comprises a pair of deflector plates positioned opposite each other across the beam path. The deflector plates will deflect the electron beam to an extent proportional with the magnitude of the deflecting voltage applied therebetween. This statement holds true, however, only when the deflecting voltage magnitude remains the same during the passage of the electrons through the deflecting electric field. The deflector plates will be incapable of providing the desired beam deflection if the frequency of the deflecting voltage is so high that the voltage magnitude changes during the passage of the electrons through the deflecting field.
A knonw solution to this problem is the traveling-wave deflection system comprising a deflecting waveguide of helical or other configuration which functions as a kind of delay circuit. The traveling speed of the electrons in a beam is approximately synchronized with that of the deflecting voltage from one end to the other of the waveguide. Thus, as the deflecting voltage acts on the electron beam for a longer period of time, desire deflection can be provided over a wide frequency range of the deflecting signal.
Japanese Patent Publication No. 57-10539, dated Feb. 26, 1982, represents a conventional device based on this priciple. This prior art device comprises a pair of opposed deflectors each having a helical conductor wound around an elongate core structure of electrically insulating material. We object to this conventional device because of the almost unavoidable electrification of the insulating core structure. The insulating core structure when electrified gives rise to deflection defocusing and bright line displacement.
Japanese Utility Model Publication No. 52-13885 proposes to reduce the electrification of the insulating core structures by creating grooves therein and hence by separating their exposed surfaces from the helical conductors. We again object to this known solution because the complete surfaces of the core structures facing the beam path cannot possibly be separated therefrom by creating grooves therein. Moreover, the core structures suffer a substantial decrease in mechanical strength if the grooves are cut therein deep enough to virtually eliminate the possibility of their electrification.